Aircraft configuration

ABSTRACT

An aircraft comprising an aeroengine and a noise reflective surface, the aeroengine capable of generating a hot exhaust jet and noise characterised in that the reflective surface is profiled to reflect noise from the aeroengine into the hot exhaust jet thereby attenuating reflected noise.

The present invention relates to a configuration of a surface of anaircraft, adjacent an aeroengine, for noise attenuation of theaeroengine.

Noise generated by aeroengines is a significant proportion of theoverall noise generated by an aircraft. As more stringent environmentalnoise pollution legislation comes into effect it becomes more importantto reduce noise by previously uneconomic airframe configurations.

One source of noise pollution from the aircraft is noise generated bythe engine, which is then reflected from an aerofoil surface of theaircraft such as a fuselage, tailplane or wing. The engine noise may beeither from the engine itself or from the exhaust jet.

In the paper, “Model Tests Demonstrating Under-wing Installation effectson Engine Exhaust Noise”, AIAA-80-1048, Way, D. J. & Turner, B. A.,1980, it is recited that reflection of the jet noise from the wingunder-surface is evident, but is less than predicted. The reducedreflected noise is believed to arise principally from attenuation by theexhaust jet as the reflected noise passes through the jet exhaust.

Furthermore, in “Wing Effect on Jet Noise Propagations” AIAA-80-1047,1980, Wang, M. E. acknowledges that the engine noise is reflected by theunderside of the wing and part of which is then refracted and attenuatedby the jet exhaust. Wang proposes a number of measures to reduce noiseenhancement due to wing effects, firstly the jet engine should bepositioned so that the major source distribution of the jet noise wouldbe downstream of the trailing edge of the wing and secondly surfacetreatment of the underside of the wing. However, positioning the enginetoward the trailing edge of the wing to avoid reflected noise wouldcause the engine to receive undesirable airflow from the underside ofthe wing and suffer an interference drag penalty. Furthermore, an enginemounted rearward of the wing trailing edge would need to be sufficientlyfar back that in the event of a rotor burst, fragment trajectories wouldnot pass through the wing. Surface treatment of an aerofoil surfacewould cause a negative impact on the aerodynamic performance of theaircraft.

Therefore it is an object of the present invention to provide anaircraft comprising an aeroengine and a noise reflective surface, theaeroengine capable of generating a hot exhaust jet and noisecharacterised in that the reflective surface is profiled to reflectnoise from the aeroengine into the hot exhaust jet thereby attenuatingreflected noise.

Preferably, the reflective surface comprises a substantially arcuateprofile with respect to a plane normal to the aeroengine centre line.

Preferably, the arcuate profile is generated by a radius from the enginecentre-line. Alternatively, the arcuate profile is generated by a radiusfrom below the engine centre-line.

Alternatively, the reflective surface comprises a substantially arcuateprofile that extends between the leading edge of the wing to thetrailing edge of the wing.

Alternatively, the noise is generated by the engine. Alternatively, thenoise is generated by the mixing of the exhaust jets issuing from theengine and the ambient air.

Preferably, the reflective surface is that of a wing, alternatively thereflective surface is that of a fuselage.

The present invention will be more fully described by way of examplewith reference to the accompanying drawings in which:

FIG. 1 is a schematic section of part of a ducted fan gas turbine engineand an aircraft wing in accordance with the present invention;

FIG. 2 is a rear view of a wing mounted gas turbine aeroengine.

With reference to FIG. 1, a ducted fan gas turbine engine generallyindicated at 10 has a principal and rotational axis 11. The engine 10comprises, in axial flow series, an air intake 12, a propulsive fan 13,an intermediate pressure compressor 14, a high-pressure compressor 15,combustion equipment 16, a high-pressure turbine 17, and intermediatepressure turbine 18, a low-pressure turbine 19 and a core nozzle 20. Acore duct 22 is partly defined radially inwardly by a core plug 23 andradially outwardly by the core nozzle 20. A nacelle 21 or other aircraftarchitecture generally surrounds the engine 10 and defines the intake12.

The gas turbine engine 10 works in the conventional manner so that airentering the intake 11 is accelerated by the fan 13 to produce two airflows: a first air flow into the intermediate pressure compressor 14 anda second air flow which passes through a bypass duct 24 to a bypassexhaust nozzle 34 to provide propulsive thrust in the form of agenerally annular bypass exhaust jet 31. The intermediate pressurecompressor 14 compresses the air flow directed into it before deliveringthat air to the high pressure compressor 15 where further compressiontakes place.

The compressed air exhausted from the high-pressure compressor 15 isdirected into the combustion equipment 16 where it is mixed with fueland the mixture combusted. The resultant hot combustion products thenexpand through, and thereby drive the high, intermediate andlow-pressure turbines 17, 18, 19 before being exhausted through thenozzle 20 to provide additional propulsive thrust in the form of agenerally circular core engine exhaust jet 30. The high, intermediateand low-pressure turbines 17, 18, 19 respectively drive the high andintermediate pressure compressors 15, 14 and the fan 13 by suitableinterconnecting shafts 25, 26, 27.

The fan 13 is surrounded by a structural member in the form of a fancasing 28, which is supported by an annular array of outlet guide vanes29. A pylon 32 extends from an aircraft wing 33 and attaches the engine10 to the wing 33 in conventional fashion.

Where an aero-engine 10 is installed under an aircraft wing 33, or belowa tail-plane assembly, the aerofoil underside surface 35 provides asubstantial reflective surface 35, which increases the noise level belowthe aircraft. However, the engine's core exhaust jet 30 partiallydisperses and attenuates the reflected signal, reducing the reflectednoise increase apparent below the aircraft. The dispersion of reflectednoise is caused by refraction of the sound passing through the core jet30, which has a higher speed than the bypass and ambient streams and issubstantially circular in cross section. The attenuation is attributedto unsteadiness in the heated flow impeding the acoustic propagation ofsound for noise having a wavelength that is small relative to thediameter of the jet 30.

The present invention comprises profiling the underside of the wing 33,or other reflective aircraft aerofoil surface 35, to reflect a greaterproportion of the reflected noise into the core jet 30. FIG. 2 bestshows a suitable profile 36 for focussing reflected noise into the coreexhaust jet 30. The profile 36 is a defined by the arc of a radius Rfrom the engine centre line 11. Thus any noise generated by the engineor exhaust jet will radiate from the centre-line and be reflected backsubstantially along the same radial path when looking at FIG. 2.

Profile 36 is an ideal profile considering only focussing reflectivenoise into the centre of the core exhaust jet 30. However, whenconsidering also the aerodynamic performance of the wing 33 a shallowerprofile 37 is preferable. The shallower profile 37 is still capable ofreflecting noise into part of the core exhaust nozzle jet 30 and thusremains beneficial to reducing reflected noise from the underside of thewing 33.

A benefit of the under wing profile 36, 37 is that there is an increasegully depth, that is the distance between the engine and the wing. Thisincreased gully depth is advantageous in that there is either lessaerodynamic interference drag for a given engine size or a larger enginewith an increase fan diameter may be used with no aerodynamicinterference drag reduction.

The profiling should be such that the incident sound rays emanating fromthe engine 10 and jet mixing 38, 39 noise sources are reflected backtowards the hot jet. The resulting concave profiling 36, 37 has an axialextent D along the wing chord such that the engine 10 and jet mixing 38,39 sources are reflected back to the hot jet for as much of the wingchord as practicable. In FIG. 2, the extent D of the profiled surface36, 37 is substantially from the leading edge 41 of the wing 33 to thetrailing edge 42. The profile extends from the trailing edge 41 as somenoise is radiated towards this region and is then reflected towards andinto the core exhaust stream 30 as the aircraft and engine travelforward out of the way of the reflected noise.

It is not necessary for the profile 36, 37 shown in FIG. 2 to be partcircular, merely that the profile is capable of reflecting noise intothe core or hot exhaust jet 30. Thus other arcuate profiles, such as anellipse or a combination of dihedral and anhedral, are substitutablewithout departing from the scope of the present invention.

As shown in FIG. 1 the profiled under wing extends rearward to a wingflap 40 of the wing 33. However, the profile 36, 37 extendssubstantially between the leading edge 41 and the trailing edge 42 ofthe wing. The profile 36, 37 is generally arcuate, in respect of thesection shown in FIG. 1, and is capable of directing the reflected noiseinto the core exhaust jet 30.

Considering the air flow in this region, the effects of aircraft motion,the engine and jet noise source distribution as well as other mechanismsaffecting the acoustic propagation, the aircraft geometry—for instance,wing dihedral, and the observer position relative to the individualengines on the aircraft, the optimised design may vary from thissimplified case. For instance, the radius of the concave surface R mayvary both along the axial extent, D, and across the width-wise extent,L. L may also vary along the axial extent D as the radial extent of thecore exhaust jet 30 increases. The axial extent

For engines mounted under an aircraft tailplane, the underside of thetailplane surface should be profiled over an appropriate extent L toenable reflected noise reduction on the ground over a significantregion. The radius R can vary azimuthally and axially along the lengthof the wing to optimise the hot jet dispersion and attenuation of theacoustic reflection in balance with the aerodynamic and structuralconsiderations. It may be possible to extract aerodynamic advantage fromthis invention as a result of enabling the engine to tailplane undersidespacing to be increased and to vary less with azimuthal angle, than withconventional designs.

Whilst endeavouring in the foregoing specification to draw attention tothose features of the invention believed to be of particular importanceit should be understood that the Applicant claims protection in respectof any patentable feature or combination of features hereinbeforereferred to and/or shown in the drawings whether or not particularemphasis has been placed thereon.

1. An aircraft comprising an aeroengine and a noise reflective surface,the aeroengine capable of generating a hot exhaust jet and noisecharacterised in that the reflective surface is profiled and positionedto reflect noise from the aeroengine into the hot exhaust jet therebyattenuating reflected noise from the profiled reflective surface.
 2. Anaircraft as claimed in claim 1 wherein the reflective surface comprisesa substantially arcuate profile with respect to a plane normal to theaeroengine centre line.
 3. An aircraft as claimed in claim 2 wherein thearcuate profile is generated by a radius from the engine centre-line. 4.An aircraft as claimed in claim 2 wherein the arcuate profile isgenerated by a radius from below the engine centre-line.
 5. An aircraftas claimed in claim 1 wherein the reflective surface comprises asubstantially arcuate profile that extends between the leading edge ofthe wing to the trailing edge of the wing.
 6. An aircraft as claimed inclaim 1 wherein the noise is generated by the engine.
 7. An aircraft asclaimed in claim 1 wherein the noise is generated by the mixing of theexhaust jets issuing from the engine and the ambient air.
 8. An aircraftas claimed in claim 1 wherein the reflective surface is defined by awing.
 9. An aircraft as claimed in claim 1 wherein the reflectivesurface is defined by a fuselage of the aircraft.